1. Field of the Invention
The present patent application relates to the field of inkjet printing, and particularly to inkjet printing for patterning in the manufacture of displays and electronics. Further, the present invention relates to an inkjet print head and a method for increasing droplet placement accuracy in such a print head.
2. Description of the Related Art
Most inkjet print heads used today have circular nozzle orifices. An ejected ink droplet usually has a tail, which in the ideal situation should be released from the center of the fluid. However, because there is a dynamic motion of the fluid in a fluidic column of the print head, a situation occurs which renders it unfavorable for the ink droplet to release from the center of the fluid. In the above described situation the release point of the ink droplets tail is probably located at the edge of the nozzle orifice. Due to the above described prior art orifices being circular, there is no particular reason for a tail of a droplet to favor one location or another on the periphery of the orifice to make its final departure from. Thus, this leads to the possibility of the tail break-off varying randomly from one side of the orifice to another due to the dynamic motion of the fluid in the fluidic column of the print head. This phenomenon might influence the trajectory of the ejected ink droplet, such that the ink droplet is expelled improperly and is delivered to an undesired location on the print media material, e.g. substrate. The place where the tail of the ink droplet is most likely to attach is the roughest edge of the nozzle orifice, which position can be located anywhere on the edge of the nozzle orifice, and can change over time as a result to unintended damage of the nozzle edge. This will cause a random deviation in the straightness of the droplet trajectory relative to a center line of the nozzle. This variation in tail break-off position is likely to lead directly to dot-placement errors.
Inkjet patterning technology can be used in the manufacture of so called polyLED color devices. The straightness of the droplet trajectory is a very important factor in ensuring high print quality in such applications. As an example, a typical ink droplet can have a diameter of 30 micrometers in flight and approximately 40 micrometers when landed on the print media material, e.g. substrate. Currently pixels of so called polyLED devices are approximately 50 to 60 micrometers wide, which leaves an error margin in pixel placement of only 5 to 10 micrometers. Taking the above into account, this means that the straightness of the droplet trajectory, i.e. the intended pathway to be followed by the ink droplet in order to created the final printed image pattern, is a very important factor in such patterning. In the case of patterning Light Emitting Polymers the interaction of polymer chains will cause a long filament tail as the droplet falls. This filament tail is likely to further influence the deviation of the ejected droplet from a direction along the center line of the nozzle when it does not hang from the middle of the fluidic column in the nozzle but on a non-defined part of the edge of the nozzle orifice. As mentioned above, this position can vary from nozzle to nozzle resulting in an intrinsic error, which cannot be influenced by changing the printing settings. In the case of patterning Light Emitting Polymers this might result in insufficient spacing between adjacent pixels.
Previously known patent publication U.S. Pat. No. 6,299,289 relates to an inkjet print head, typically constructed by a MEMS (Micro Electro Mechanical System) process, which includes a large number of nozzle chambers with ink ejection nozzle apertures and a paddle moveable in each chamber by an actuator to deliver ink through the nozzle. Each paddle is provided with a projecting poker, concentrically protruding through the nozzle aperture when actuated to inhibit clogging of the nozzle.
Although it is envisaged that a return movement of the actuator will result in a general necking and breaking of a meniscus around the poker to form a droplet a drawback of the above described print head according to U.S. Pat. No. 6,299,289 is that a filament tail of the droplet is likely to attach to an undefined position at the periphery of the poker, which, in the same manner as described above, is likely to cause a random deviation of the ejected droplet from a direction along the center line of the nozzle as the poker will need to be of a certain dimension in order to perform the intended function of inhibiting clogging of the nozzle. Further, as is evident from the drawings of U.S. Pat. No. 6,299,289, the peripheral portion of the distal free end of said poker will not be aligned with the center line of the nozzle aperture when actuated.